1. Field of the Invention
The present invention relates to a printed board having an impedance matching member between the printed board and connecting pins for connecting the printed board and a connector.
2. Description of Related Art
As the operating frequency of the electronic circuits increases, it becomes more important to provide characteristic impedance matching between circuits and between nodes in order to maintain excellent transmission characteristics of high-frequency signals and high-speed signals. Circuits, devices or components inside the devices are connected to each other using connectors and cables. It is therefore necessary that a design of signal lines between a connector and a printed board be made in view of the impedance matching. Signal lines such as microstrip lines, strip lines and coplanar lines provided on a printed board are designed to match impedances in accordance with the configuration of the signal lines and the positional relationship with a ground line. However, in some cases, connecting pins between a connector and signal lines on a printed board are exposed in the air with no ground line corresponding thereto. This makes it difficult to provide sufficient impedance matching.
For example, in a horizontal connector mounted on a printed board so that connecting pins extends horizontally, the connecting pins extending from the connector are bent at an almost right angle to be connected to the printed board and exposed in the air. Such exposed portions of the pins have high impedance, causing impedance mismatches to occur between the pins and the signal lines on the printed board. This results in a problem that transmission characteristics of high-frequency signals and high-speed signals significantly degrade due to signal loss, pulse distortion or the like caused by the impedance mismatching.
The connecting pins bent at an almost right angle have different impedances at inside and outside of the bent portions.
FIG. 12 is a cross-sectional view of a connector 14 mounted in a horizontal orientation on a printed board 11. FIG. 13 is a view of the same portion as seen from the rear side of the connector. The printed board 11 has a ground line 12 provided on one surface thereof and signal lines 13 such as microstrip lines, strip lines, coplanar lines or the like provided on the other surface. The connector 14 has a shield case attached thereto near its connection to a cable. The shield case 16 is fixed to the printed board 11. The connector 14 has a connecting portion for connection to the cable and a plurality of connecting pins 15a, 15b and so on extending from the connecting portion. The connecting pins 15a, 15b and so on are soldered to the signal lines 13 of the printed board 11.
When the connecting pins 15a, 15b and so on of the connector 14 are thus connected to the printed board 11, the pins are exposed in the air except for the portions covered with the shield case 16. Such exposed portions have various impedances, and variations in impedance are greater in the outermost pin having the largest exposed portion than in the inner pins. Particularly, there is a variation in common mode impedance (impedance with respect to the ground line), and thereby common mode reflection occurs.
Japanese Unexamined Patent Publication No. 2002-270307 solves such problems by disposing a shielding member near the exposed portions of the connecting pins 15a, 15b and so on to prevent impedance mismatching.
The structure disclosed by the aforementioned patent publication has the shielding member disposed inside the connector, which limits portions where the shielding member can be disposed and makes molding of the shielding member complicated because the shape of the shielding member needs to conform to the shape inside the connector. This results in production of waste during the preparation of shielding member, an increase in time for attachment of the shielding member and an increase in cost.